The invention relates to a receiver of a system for transmitting data symbols at a given baud-rate 1/T, this receiver comprising a symbol detector operating at the baud-rate 1/T; means for optimally conditioning the received data symbols for symbol detection; means for deriving from the conditioned signal an error signal which is representative of a phase difference between a baud-rate clock signal associated with the received data symbols and a locally generated baudrate clock signal for the symbol detector; and phase control means for reducing this phase difference step-by-step in response to the error signal.
A data transmission system with such a receiver is known from the U.S. Pat. No. 4,494,242. In the known receiver the received data symbols are sampled at the baud-rate. As indicated in this patent, such a sampling is advantageous among other things in that the required equipment can be considerably more simple than when sampling at the higher Nyquist rate.
When sampling in the receiver of a data transmission system it is essential that the clock signals used in the receiver for the sampling be always in phase with the clock signals which are associated with the transmitted and received signal. For combating the unavoidable phase differences between these clock signals, the clock signal in the receiver must be continuously adjusted.
The phase adjustment in the known receiver is based on the empirical fact that the shape and size of the precursor of a received data symbol substantially only depend on filters in the transmitter and receiver and are practically independent of the properties of the transmission line between transmitter and receiver and thus possess for a specific transmission system a variation which is known in advance. Now, by comparing a fraction of the instantaneous value of the received signal to the precursor portion of that signal at the previous sampling instant, information can be derived about the phase shift between the detection instant proper and the desired detection instant, which information in the known arrangement is used for adjusting the phase.
In order to avoid errors in the symbol detection, it is desired that the phase can be adjusted accurately. The latter requires a phase control by phase steps having a sufficiently small size and thereto a high internal clock rate should be used in the receiver.
The requirements which are made on the accuracy of the phase control are even stricter if full-duplex data transmission through a single wire pair is concerned In full-duplex data transmission, as is well known, a compensation signal is used which is obtained by means of echo cancellation (EC) either in combination or not in combination with decision feedback equalization (DFE). In this case, a relatively small phase step may already lead to a considerable adjustment of the adaptive arrangements supplying this compensation signal, which adjustment may take several hundreds of symbol intervals, so that the probability of errors in the symbol detection is increased considerably. This is especially a problem if, in response to the phase control, the phase keeps fluctuating in small steps around the optimum final value and if such a small step causes a change of the respective coefficient values in the echo canceller (EC) and the decision feedback equalizer (DFE).
By the choice of a sufficiently high clock rate resulting in the possibility to make small phase steps, this problem can be overcome. However, to this end use must be made of a relatively costly clock signal regenerator. Moreover, too high chock rates are undesired in view of the maximum processing rate of digital components in the receiver, whilst also inadmissibly high power dissipation may occur at these high rates.
In the article "A long reach digital subscriber loop transceiver, by P. F. Adams et al., in British Telecommunication Technology Journal, Vol. 5, No. 1, Jan. 1987, pp 25-31, a different solution is described, in which an echo canceller is used comprising means for estimating the required deviation of the echo compensation signal in the case of a phase step. Also according to this article, the phase deviation is estimated while using the fact that the precursor value is substantially independent of the transmission line. The value of the precursor, however, is estimated by reducing the value of the signal sample close to the peak of the received data symbol by a constant number of times the value of the signal sample which was determined T seconds earlier. Such an echo canceller too is intricate and expensive and, consequently, preferably not used.